Ahmet

tecart, I'd be interested in hearing more about this situation if you don't mind sharing.
Thanks
Ahmet

David Floyd

Tecart, fill us in, I think alot of guy's have this problem.

David Salama

Tecart,

I wont contact you directly since everyone wants to know your secret. Please share !!

tecart

Well ill post to all then, the bottom line is their is supposed to be vaccume not pressure at idle at the oil fill cap. If you undo this cap and put your hand over the hole and you feel air pressure of some signifigance your car has a problem. To prove there maybe a problem, along with this pressure your car maybe leaking oil from either the rear main seal/ cork cam housing gasket or from the oil pan or front main seal... Also your car may not be at the next damaged stage but that goes into smoking hard and loss of performance while on boost and the feeling like you may be running on 3cycl. Well i had a professional look into this, and he told me these cars are supposed to only have vaccume at idle, nothing else. Now the problem can be created, typically after a motor rebuild. The crankcase girdle may not of been sealed or torqued down properly, if not torqued down one of the bearings may not be touching the crank at all or a piston may have been intalled backwards, thus putting side load on the piston skirt "welding its material onto the cylc. wall", and causing the piston ring to be sealed over with piston material. Also you may have a bent intake valve which is what i had. These were all the things he mentioned. A compression test/ leakdown test will show you for sure if their is a problem, usually being an excess buildup of crankcase pressure. Did you do the pressure checks according to the factory specs? I think if you got positive pressure at the fill tube then your pressure checks will be bad. My number 2 showed a huge drop. The head came off to reveal a bad valve and 2 sets of bad rings, and also the crankcase girdle wasnt torqued down right. After the above was fixed, i have vaccume at idle, no leaking, and a smoother running car. Also before the car was checked it ran very hard, even with crankcase pressure and a small rear main leak, but this is a cause for concern/ red flag. I luckily had the head/ valve and rings fixed intime. This was all after a bad mechanic overlooked the basics on the first rebuild. Just to let you know their should be vaccume, and that usually means their isnt a hidden red flag that may bite you later.

TonyG

I've got good leak down results, and I don't have vacuum at idle. In fact, I've never seen a 951 that did. None of the 3 951's and my 968 don't have vacuum at idle at the oil filler cap.

Not a lot of air coming out, but definitely not vacuum.

Where exactly would this vacuum be coming from? The pistons don't seal perfectly resulting is some blow by. This is a fact. So where is the crank case supposed to be getting a vacuum source from?

The breather system connects to the intake tract in front of the throttle body, so there's no real vacuum being produced at idle at the other end of the line.

I'm interested.

Please enlighten us all.


Thanks,

TonyG

Also...

Regarding the engine rebuild scenario...


Even is a bearing wasn't touching a crank (which I find extremely difficult to believe), how would this in any way create vacuum (or pressure for that matter) in the block?

Thanks again.. (not intended as a flame BTW)

tecart

My crank girdle wasnt tightened and torqued down 100% to specs., upon inspecting the new rods and mains, one of the bearings looked untouched, like new out of the package, the rest had normal wear to them, my mechanic said that one of the bearings was not touching the crank the way it is supposed to due to the previous mechanic improper torque job, he also did not find any sealant remnants on the girdle, like he was supposed to, this was helping create oil leakage to the real main seal. Now as to the pressure at idle, i had alot of it, like if you put paper over the cap it would blow it out of the way fast, their should be no excess pressure, if you feel nothing thats ok as the turbo will suck out some excess pressure but in most cases the test will show a sucking in rather than blow out senerio if all is well. Call your local dealer and ask him if their should be pressure or vaccume at idle at this spot and you will get the same answer i got, at first i tought maybe all the 944's do this, but then he said this is a problem, and the leakdown test next confirmed this, then with the head off, a bad valve he finds, then we decide to look at the bottom end, and he finds 2 more problems. It seems that my car went right along with the pressure test so for now thats something im gonna check often to be safe.

TonyG

I can see how 911's will have vacuum at idle. Both mine did.


But as I said before, there's simply no source for vacuum in the crank case on a 951 (whereas on a 911 the PVC is connected to the intake manifold where vacuum always persists). The PVC is connected to a location in front of the turbo, which isn't going to produce vacuum (maybe a tiny bit from the venturi effect.. but not enough to offset the leakage from the cylinders).

If you're getting vaccum now, I'd be real interested to know how and where it's coming from.

With my 951's if you remove the oil filler cap at idle, you get a pulsing feeling when you put your hand over the filler (which again.. is the same for all my 951's).


I'm perplexed! :-)

tecart

you get a pulsy feeling, mine was more like the back of an exhaust pipe feeling but not that severe but close

turboven

I got my atmosphere blow-off valve back in 1996 which I bought from Performance Racing in Tulsa as a "IMSA type turbo boost valve", and it doesn't look like the one that David pictured.
Mine is much bigger and has spring and bolt to control blow-off pressure and for sure it doesn't let fresh air in, it only opens when reaches @1.2 bar.
And let me tell you guys that the difference is huge as when I swiched from the original.
I may post a picture if you request it.

Regards,

Bruno.

superjet.1

Well i have to put in my two cents (as usual).It seams people thank the 944s and 951s need vacuum at idle or are suppose to have vaccum this is not the case.The crank case has no vacuum being pulled from it it only has air being moved around by the pistons and crank thus the pulsing affect mentioned. witch is the piston pulse moving up and down.Now if you have large quantities of air coming out the breather at all times than you have a leak down problem threw the rings ONLY this is the only place air can get from the combustion chamber to the case. bent valves will only allow air either back up the intake or out the exhaust unappeaded.